Fetal rats may recover virtually completely or nearly completely from severe radiation damage at certain stages. Higher doses cause malformation, one pattern for example being stenosis of the aqueduct with hydrocephalus. How injured neuroepithelial cell populations organize themselves to recover after on level of injury, but fail at a higher level is being studied by light and electron microscopy, and immunocytologic methods. A mutant rat that imitates the radiation-induced aqueduct stenosis is compared to the irradiated rats. Neuroepithelial-basement membrane-mesenchyme relations are on focus of study. The possibility of delayed radiation effects on astrocyte development and the possible role of macrophages in aggravating malformation are being studied. The health related aspects of the work are 1) that mammalian fetuses can recover remarkably at certain stages from severe injury, 2) mechanisms that underly the development of prenatal aqueduct stenosis are being investigated, and 3) possible delayed effects of radiation on developing cell populations, and the role of macrophages in fetal injury are being studied.